1. Field of the Invention
This invention relates to a method for improving reinforced intermetallic matrix composites, and particularly to modifying ceramic fiber reinforced molybdenum disilicide matrices so as to improve high temperature strength, creep resistance, and toughness.
2. Description of the Prior Art
Molybdenum disilicide is an intermetallic compound with a melting temperature in excess of 2000.degree. C., excellent high temperature oxidation resistance, and high thermal conductivity. Several problems, however, limit the use of molybdenum disilicide as a high temperature material, such as insufficient high temperature strength, creep resistance, and toughness. Accordingly, fiber reinforcement of molybdenum disilicide has been attempted, using high strength continuous ceramic fibers such as aluminum oxide and silicon carbide. During thermal cycling, however, ceramic fiber reinforced molybdenum disilicide matrices experience cracking caused by thermal stresses resulting from differences in the coefficients of thermal expansion of the matrix and the reinforcing fiber. As disclosed herein, applicants have now found a method to form a ceramic fiber reinforced molybdenum disilicide composite capable of withstanding such thermal stresses, by adding to the matrix a particulate material which modifies the coefficient of thermal expansion thereof, to a value close to that of the reinforcing fiber.
Attempts have been made previously to improve the high temperature capability of molybdenum disilicide matrix materials, such as by addition of silicon carbide whiskers. For example, Petrovic et al, in U.S. Pat. No. 4,927,792, disclose a molybdenum disilicide matrix composite which is reinforced with SiC whiskers throughout the matrix, to improve strength at high temperatures. The patentees' approach to overcoming matrix cracking during thermal cycling is to have the fibers in tension, and the surrounding matrix under compression, thus requiring a relatively high density of uniformly spaced fibers. Petrovic et al do not, however, suggest the inclusion of particulates to reduce the differences between the coefficient of thermal expansion of the MoSi.sub.2 matrix and the reinforcing fibers.
Petrovic et al, in U.S. Pat. No. 4,970,179, disclose a modified MoSi.sub.2 alloy matrix composite wherein the matrix contains from about 10 to about 30 percent SiC in the form of whiskers or submicron powder. In order to achieve increased strength at high temperatures, a portion of the MoSi.sub.2 in the matrix is replaced with one or more refractory metal silicides, selected from tungsten disilicide, niobium disilicide, tantalum disilicide, molybdenum trisilicide, tungsten trisilicide, etc. Petrovic et al do not, however, suggest the inclusion of particulates to reduce the differences between the coefficient of thermal expansion of the MoSi.sub.2 matrix and reinforcing fibers, and in fact do not suggest the inclusion of continuous ceramic reinforcing fibers to strengthen the matrix.
Washburn, in U.S. Pat. No. 5,045,237, discloses a refractory electrical device for use as a heating element, ignitor, and heat sensor, which contains fine powders of molybdenum disilicide, silicon carbide, and aluminum nitride which are sintered or hot pressed into rigid structures. The patent does not teach, however, the use of ceramic reinforcing fibers.
Agarwal et al, in U.S. Pat. No. 4,935,118, disclose a self-heated oxygen sensor package having a heating element comprising silicon carbide, silicon nitride, or molybdenum disilicide, or mixture thereof. The reference teaches the addition of silicon nitride to avoid false readings of oxygen content, but makes no disclosure of modifying thermal expansion coefficients or adding ceramic reinforcing fibers.
Schrewelius, in U.S. Pat. No. 4,016,313, discloses a heat resistant material for use in kilns, and attempts to overcome decreased strength due to oxidation by filling the pores of the silicon carbide matrix material with an impregnate containing molybdenum disilicide and silicon. The reference, however, does not attempt to strengthen the matrix by the addition of ceramic fibers, or to modify the thermal expansion coefficient of the matrix.
In summary, while the prior art has disclosed the addition of particulate materials to molybdenum disilicide matrices to modify the high temperature properties thereof, and the use of reinforcing fibers in matrices, the references have not taught the modification of the thermal expansion coefficient of the matrix to reduce stress between the matrix and a continuous ceramic reinforcing fiber encompassed therein. Thus, the references have not overcome the problem of stress induced by thermal cycling of ceramic fiber reinforced molybdenum disilicide matrices.